A relatively recent innovation in boatlifts includes the use of hydraulic cylinders to raise and lower boats. In one such boatlift, a cradle for carrying a boat is mounted between opposed pilings. A pair of hydraulic cylinders with horizontally extensible rods are mounted in housings affixed to the pilings on either side of the cradle. The rods are connected to the cradle by cables routed through a plurality of sheaves such that horizontal motion of the rods are translated into vertical motion of the cradle. An example of this type of boatlift is described by U.S. Pat. No. 7,413,378, the contents of which are hereby incorporated by reference in their entirety.
In areas with substantial local variations in water level, such as tidal changes, the required lift cradle travel distance can vary substantially. For example, the required lift cradle travel distance at low tide can be several feet greater than at high tide. As a result, a significantly more complex and robust arrangement of hydraulic cylinders and sheaves may be required to accommodate the lift cradle travel distance at low tide than are needed at high tide.